Liquid additive reservoir for selective catalytic reduction system

ABSTRACT

A liquid additive reservoir including a rigid casing delimiting a housing, and a fluid-tight flexible pouch mounted inside the housing of the casing and intended to contain the liquid additive, the flexible pouch being designed to be connected to the exhaust line of an internal combustion engine of a vehicle.

TECHNICAL FIELD

The present invention relates to a liquid additive reservoir for a selective catalytic reduction system, to a selective catalytic reduction system comprising such a reservoir and to a method of filling such a reservoir.

BACKGROUND

Standards aimed at reducing pollutant emissions generated by motor vehicles are becoming ever more severe, and are constantly forcing motor vehicle manufacturers to evolve their technologies in order to comply with these standards. Such standards are aimed amongst other things at reducing the discharges of nitrogen oxides (NOx) into the atmosphere.

In order to limit nitrogen oxide emissions, motor vehicle manufacturers are increasingly resorting to the SCR (Selective Catalytic Reduction) treatment system which allows nitrogen oxides to be reduced through a reducing agent, such as ammonia, in the exhaust line of the vehicle internal combustion engine. This ammonia generally comes from thermolytic decomposition of an aqueous solution of urea, for example of AdBlue®, injected directly into the exhaust line. This aqueous solution of urea may be enriched with various components, such as ammonium formate, as is the case in Denoxium®.

An SCR treatment system as is known comprises a reservoir containing an aqueous solution of urea, fixed to the vehicle, a NOx filter situated on the exhaust line, and an aqueous-urea solution injection device connected to the reservoir and designed to inject the solution of urea into the exhaust line of the engine upstream of the NOx filter.

In such a system, the aqueous solution of urea injected into the exhaust line is converted into ammonia because of the high temperature in this exhaust line. The ammonia thus obtained then reacts, in the NOx filter, with the nitrogen oxides resulting from combustion in the engine so that these oxides are converted by catalytic reduction into nitrogen, which presents no danger to the environment, and water vapor.

The reservoir of such an SCR treatment system is commonly made up of a rigid one-piece shell or of two rigid half-shells joined together, and is generally fitted with a vent valve to compensate for in-reservoir pressure variations associated in particular with expansion, evaporation, and the drawing off of the solution contained in this reservoir.

A reservoir such as this has a significant number of disadvantages.

Specifically, the reservoir vent valve is liable to release ammonia vapors and water vapors, either because this valve seals imperfectly or simply when the pressure in the reservoir rises above the rated pressure of the valve, for example in high summer when exposed to hot temperatures causing ammonia vapors and water vapors to form.

Such gaseous discharges on the one hand contaminate the environment because they contain ammonia and, on the other hand, alter the concentration of the aqueous solution of urea, this being something that could disrupt and impair the operation of the SCR treatment system.

Furthermore, as it dries out, the urea solution generates solid deposits, particularly of urea. These deposits may form at the vent valve and jam it in the open position (in which case there will be very significant vaporization losses) or in the closed position. If the vent valve becomes jammed closed, either it will no longer be possible to draw the urea solution from the reservoir or the reservoir will break as such withdrawal causes it to collapse.

The presence of a vent valve also makes it difficult to supply such a reservoir prefilled, because of the risks of leakage.

The geometry of such a vessel needs to be determined precisely in order, for example when the vehicle is inclined or in motion, to prevent the risk of air rather than urea solution being sucked up, especially when the reservoir has been partially emptied. What this means is that the geometry options for such a vessel are limited, and that the working volume of fluid (which will definitely be used without generating problems with sucking up air in particular) is small by comparison with the total volume of the reservoir and by comparison with the space available in the vehicle to accommodate this reservoir. It is therefore an awkward matter to install on a vehicle a reservoir that has a high working volume, and this places a burden on the user of the vehicle who has to keep spare liquid additive available or have to return his vehicle to the dealership prematurely.

In addition, the gauging of such a reservoir is somewhat inaccurate, especially because of the complex geometry of the reservoir, and it is expensive. The gauging of such a reservoir may in particular be influenced by phenomena whereby the reservoir is placed under pressure/depression, connected with the opening pressures of the vent valve.

It should be noted that present-day SCR treatment systems operate predominantly on a solution of urea (AdBlue®) which freezes at around −11° C. This particular characteristic entails the provision of means of heating the reservoir, or even of means of heating the circuits that carry urea solutions. Furthermore, in order to limit the risks of the urea solution freezing, numerous manufacturers provide a purge circuit to prevent the urea solution from freezing in the pipes when the vehicle is stationary. Such arrangements significantly increase the costs of manufacture of the SCR treatment system.

BRIEF SUMMARY

It is an aim of the present invention to overcome all or some of these disadvantages.

To this end, the invention relates to a liquid additive reservoir for a selective catalytic reduction system intended for treating the exhaust gases of an internal combustion engine of a vehicle, characterized in that it comprises a rigid casing delimiting a housing, and a fluid-tight flexible pouch mounted inside the housing of the casing and intended to contain the liquid additive, the flexible pouch being designed to be connected to the exhaust line of the engine.

As the liquid additive is gradually consumed, the flexible pouch deforms reducing its interior volume. As a result, the reservoir according to the invention requires no conventional air vent system nor does it have the aforementioned disadvantages inherent to this air vent system, the flexible pouch being initially filled without any air and thus remaining free of air throughout its life in the vehicle.

Advantageously, the flexible pouch is mounted removably in the rigid casing. As a result, once empty, the flexible pouch can be quickly and easily removed from the flexible casing and be replaced by a new one or be refilled.

The flexible pouch is preferably made of thermoplastic by blow-molding. Because of the freedom of geometry offered by the blow-molding technique and because of the absence of geometric constraints associated with the risks of drawing in air (because there is no air present in the flexible pouch), it is an easy matter to produce a reservoir capable effectively of occupying the volume available in the vehicle, and therefore of installing a high-volume reservoir, for example under the body shell of the vehicle. It is therefore possible to align the frequency with which the flexible pouch has to be filled to the servicing intervals of the vehicle, thus avoiding the need for the user to keep a reservoir of liquid additive or return his vehicle to the dealership prematurely.

It should be noted that the design efficiency (corresponding to the working volume of fluid carried onboard with respect to the volume available on the vehicle) may fall as low as about 50% for conventional rigid reservoirs, whereas with the same allocated volumes, they are able to reach between 75 and 90% with reservoirs according to the invention.

It should also be emphasized that such a liquid additive reservoir comprising a rigid casing delimiting a housing, in which a fluid-tight flexible pouch made of thermoplastic by blow-molding is mounted, could be used as an additive reservoir for a particulate filter.

In that scenario, the flexible pouch would not be designed to be connected to the engine exhaust line but to be connected to the fuel reservoir of the vehicle or to the engine fuel feed line.

Furthermore, the blow-molding technique makes it possible to create reservoirs that are thin with a large surface area that are difficult to achieve in rigid form, so that these reservoirs can easily be installed in specific housings in the vehicle, for example under the body shell or in place of the spare wheel in the luggage compartment, etc.

This freedom of geometry also means that several reservoirs can be positioned on the vehicle far more simply, which reservoirs may either feed into a main reservoir or form series or parallel supplies directly to a pump coupled to a liquid additive supply line of an SCR system.

According to one embodiment alternative, the flexible pouch could be made from two flexible films welded together. Advantageously, the flexible films are made of thermoplastic polyurethane (TPU). For preference, the welded films are welded together by thermowelding.

According to another embodiment alternative, the flexible pouch could be made from two flexible half-shells welded together, the two half-shells being made for example by injection-molding or thermoforming. Such an embodiment of the flexible pouch makes it easier to introduce special devices into the pouch, such as a plunger that allows the liquid additive to be drawn from a specific region of the pouch. For preference, the two flexible half-shells are welded together by thermowelding.

The liquid additive is advantageously a liquid reducing agent designed to reduce the NOx present in the internal combustion engine exhaust gases, such as an aqueous solution of urea, for example of AdBlue® or of Denoxium®. Denoxium®, particularly thanks to the presence of ammonium formate, allows the freezing point of the urea solution to be lowered to −30° C., which in most cases means that any heating system can be dispensed with and may also make it possible to dispense with the operation of purging the pipes when the vehicle is stationary.

When AdBlue® is being used by way of reducing agent, there are a number of available options.

It is conceivable for example to heat the flexible pouch using resistive heating elements located in the casing under the pouch. The fuel return circuit can also be used to heat the pouch which, for example, would then rest on this return circuit which would be able to adopt a serpentine coil geometry to optimize the length in contact with the pouch and therefore the heating of the latter.

Nonetheless, an easier solution may be to have a secondary reservoir associated with heating means. This might, for example, be a second flexible pouch of small capacity that is far easier to heat up than the main reservoir. It may also be a conventional rigid reservoir, but of small volume (that could be standardized to minimize production costs and have to control diversity only on the main reservoir) and of simple design. These secondary reservoirs could, for example, allow the user to drive a few thousand kilometres if the main reservoir freezes. These secondary reservoirs will be resupplied from the main reservoir or reservoirs as soon as the temperature of the latter allows the reducing agent to defrost. When the vehicle is stationary, this secondary reservoir would have to remain full in order to be able to supply the feed circuit with reducing agent at the time of start-up, which would trigger the heating of this reservoir. Of course, in the case of a rigid secondary reservoir, this reservoir would have, through design, to be able to absorb the expansion of the fluid as it freezes.

In the case of combined use of a liquid additive reservoir for a selective catalytic reduction system and of an additive reservoir for a particulate filter (PF), one optimal configuration is to house a flexible pouch intended to contain the SCR reducing agent and a flexible pouch intended to receive the PF additive inside one and the same rigid casing. This configuration greatly optimizes the incorporation of the pollution-control system into a single module with highly optimized design efficiency, a single casing being used for attachment to the vehicle. A partition internal to the casing may possibly delineate the compartment intended to accommodate the SCR pouch from that intended to accommodate the PF pouch so as to prevent any contact between the two pouches. A small hatch (screwed, clipped, hinged or attached and opened by any other system) may be incorporated into the casing in order potentially to allow the PF pouch to be renewed. The devices for pumping the SCR and PF fluids may also be incorporated into this single casing and assembled by clipping, dovetail systems or any other method of attachment.

The flexible pouch is advantageously made of thermoplastic polyurethane (TPU), preferably based on ether or carbonate, made of polyethylene, of polypropylene, or of thermoplastic polyamide.

According to one alternative form of embodiment of the invention, the flexible pouch is of variable thickness. The flexible pouch could, for example, have a first wall portion and a second wall portion, the first wall portion having a greater thickness than the second wall portion so that the first portion is non-deformable or can be deformed only when the amount of liquid additive inside the pouch is very small.

According to an alternative form of embodiment of the invention, the flexible pouch comprises a multilayer wall. By way of example, it is conceivable for the flexible pouch to have an internal layer that optimizes resistance to the reducing agent contained in the pouch and minimizes the migration thereof into the thickness of the pouch and an outer layer that affords the pouch the necessary flexibility and ability to resist the external environment. Proliferation of layers may also be driven by cost reduction.

For preference, the casing is produced by blow-molding at the same time as the flexible pouch.

Advantageously, the flexible pouch has a neck fitted with means of connection to a liquid additive supply line of the selective catalytic reduction system.

For preference, the neck of the flexible pouch is positioned in the lower part of the pouch. Assuming that the casing is blow-molded with the flexible pouch, it is possible to incorporate into the neck a screw thread or a profile comprising a boss to allow a coupling to be screwed on or fitted easily. Advantageously, the flexible pouch comprises, in its upper part, a bulge designed to trap any liquid additive vapors (ammonia vapors, water vapors, etc.).

According to an alternative form of embodiment of the invention, the flexible pouch comprises a pipe for drawing up liquid additives, which passes in a fluid-tight manner through the neck of the flexible pouch, the end of the pipe situated on the inside of the flexible pouch being positioned near the bottom of the flexible pouch. These arrangements ensure that liquid additive will be drawn up whatever the amount of liquid additive contained in the flexible pouch, even when, under the effect of temperature, a little vapor (water vapor, ammonia vapor, etc.) is formed towards the top of the pouch. These arrangements also provide greater freedom over the positioning of the neck.

For preference, the means of connection comprise a self-closing valve or a shut-off valve.

According to one embodiment of the invention, the reservoir comprises compression means designed to apply a pressure to the flexible pouch, the compression means preferably comprising a compression spring. This compression keeps the pressure inside the pouch which will limit the generation of vapors. The design of the pouch and of the spring (travel, stiffness) will need to provide sufficient compression whatever the volume of fluid in the pouch.

According to one embodiment, the casing delimits a first housing in which at least the flexible pouch is mounted and a second housing in which an additive reservoir for a particulate filter is mounted.

The present invention also relates to a selective catalytic reduction system intended to treat the exhaust gases of an internal combustion engine of a vehicle, characterized in that it comprises a reservoir according to the invention.

For preference, the selective catalytic reduction system further comprises:

-   -   a liquid additive supply line designed to connect the flexible         pouch to the engine exhaust line,     -   an injection device coupled to the supply line and designed to         inject the liquid additive into the engine exhaust line, and     -   a selective catalytic reduction catalytic converter.

Advantageously, the selective catalytic reduction system comprises a pump coupled to the supply line and designed to carry the liquid additive from the flexible pouch to the engine exhaust line. The pump and its electronic control board may then either be incorporated into the reservoir casing, which casing will have appropriate mounting interfaces (clips, dovetails, etc.) or be offset to some other point on the supply line.

According to an alternative form of embodiment of the invention, the pump may also be coupled to the injection device (injector/pump technology).

The present invention further relates to a method of filling a reservoir according to the invention, characterized in that it comprises:

-   -   connecting the flexible pouch to a liquid additive filling line,     -   filling the flexible pouch with liquid additive,     -   measuring the pressure inside the flexible pouch or in the         filling line,     -   diverting all of the liquid additive supplied by the filling         line to a purge line when the measured pressure reaches a         predetermined threshold value.

These arrangements allow perfect control over the filling of the flexible pouch (that is to say over the amount of liquid additive introduced into it), and in particular make it possible to prevent overfilling of the pouch and damage thereof or damage to the protective casing thereof. Such control over the amount of liquid additive introduced into the flexible pouch means that the amount of liquid additive remaining inside the flexible pouch can subsequently be gauged with precision at all times, particularly from the preferred standpoint of performing digital gauging via the pump or the injection device. Specifically, starting out from an amount of fluid that is perfectly known at the time of filling, the number of revolutions of a positive-displacement pump or the time for which an injector is open, for example, can be used to relate to the amount of fluid consumed, and therefore the amount of fluid remaining in the pouch. This principle of digital gauging generally proves to be more precise and/or less expensive than other conventionally used systems, particularly envisaged designs such as flat reservoirs or reservoirs with particularly complex geometries.

This type of gauging is preferred because it offers precision which in general is highly satisfactory for a very small on-cost. The fact that precise gauging is available is very important because it makes it possible:

-   -   to carry just the amount of fluid needed, rather than providing         additional volume to absorb any potential inaccuracy in the         gauging;     -   to be alerted precisely when a minimum level prescribed by         regulation is reached, setting off an alarm warning the driver         of the need to have the product topped up.

Furthermore, these arrangements particularly allow dealerships to manage large-volume drums of liquid additive, rather than cans prefilled with the nominal fill volume of the flexible pouch. By dispensing with any manual filling operation, these arrangements very greatly reduce any risk of the reducing agent becoming contaminated during the filling phase.

Advantageously, the filling method comprises a step comprising providing a relief valve between the purge line and the filling line, the relief valve being able to move between a closed position in which the purge line and the filling line are fluidically isolated to allow the flexible pouch to be filled, and an open position in which the purge line and the filling line are fluidically connected, the valve being designed to be moved automatically into its open position when the pressure inside the flexible pouch or in the filling line reaches the predetermined threshold value.

Thus, when the flexible pouch is filled to its nominal volume, the pressure therein increases because of the resistance put up by the material to being stretched, this causing the relief valve to move into its open position and therefore purging all of the liquid additive supplied by the filling line into the purge line. Any overfilling is therefore impossible and the fact that the pouch is full can be visually checked by watching for liquid additive to flow out into the purge line.

Of course, once this filling operation has been performed, the pouch will still be able to undergo elastic deformation in order to absorb expansions of the fluid as a result of temperature or as it freezes (AdBlue). Likewise, housings created in the casing will be able to absorb these variations in volume without damage to the casing.

There are various conceivable ways of performing the aforementioned filling step. For example, it is conceivable to have a filling tank at a high level and thus to fill the pouch simply under gravity feed. Nonetheless, it may prove simpler to use a pump which can be:

-   -   either a dedicated pump on board the vehicle, which does not         appear to be a solution well optimized from an economic         standpoint, but avoids the dealership having to be equipped with         a pump device;     -   or a pump at the dealership, which appears to be the optimum         solution;     -   or the feed pump of the SCR treatment system (during the filling         step, pump operation will need to be reversed, for example by         reversing the direction in which it turns).

For preference, the filling method comprises a step comprising purging the flexible pouch before connecting it to the filling line, the purging step preferably being performed using a pump. These arrangements allow a completely empty pouch to be filled and therefore make it possible to obtain an optimum concentration and quality of liquid additive and in particular a compliance with fluid expiry dates.

BRIEF DESCRIPTION OF THE DRAWINGS

In any event, the invention will be well understood from the description which follows, with reference to the attached schematic drawing which, by way of non-limiting examples, depicts a number of embodiments of this selective catalyst reduction system.

FIG. 1 is a schematic view of a selective catalytic reduction system according to the invention.

FIG. 2 is an exploded perspective view of a reservoir according to a second embodiment of the invention.

FIG. 3 is an exploded perspective view of a reservoir according to a third embodiment of the invention.

FIG. 4 is an exploded side view of a reservoir according to a fourth embodiment of the invention.

FIG. 5 is an exploded perspective view of a reservoir according to a fifth embodiment according to the invention.

FIG. 6 is a schematic view illustrating a method of filling the reservoir of the selective catalytic reduction system of FIG. 1.

FIG. 7 is a schematic view of a selective catalytic reduction system according to a first alternative form of embodiment.

FIG. 8 is a schematic view of a selective catalytic reduction system according to a second alternative of an embodiment.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a selective catalytic reduction system 2 intended to treat the exhaust gases from the internal combustion engine 3 of a motor vehicle. The internal combustion engine 3 is preferably a diesel engine.

The selective catalytic reduction system 2 comprises:

-   -   a reservoir 4 for liquid additive, such as a liquid reducing         agent,     -   a liquid additive feed line 5 designed to connect the liquid         additive reservoir 4 to the exhaust line 6 of the engine 3,     -   an electric metering pump 7 coupled to the supply line 5 and         designed to carry the liquid additive from the reservoir 4 to         the engine exhaust line 6,     -   an injection device 8 coupled to the supply line 5 and designed         to inject the liquid additive into the engine exhaust line 6,         and     -   a selective catalytic reduction catalytic converter 9.

Advantageously, the motor that drives the pump 7 is controlled by a computer 10 to pump a required amount of liquid additive from the liquid additive reservoir 4.

For preference, the exhaust line 6 comprises an oxidation catalytic converter 11 upstream of the selective catalytic reduction catalytic converter 9, and a particulate filter 12 downstream of the selective catalytic reduction catalytic converter 9. The oxidation catalytic converter 11 has the task of treating the emissions of carbon monoxide, hydrocarbons and, to a lesser extent, particulates. Depending on the motor vehicle manufacturer strategy and the particulate filter technology used, the particulate filter 12 could also be located upstream of the selective catalytic reduction catalytic converter 9.

As shown in FIG. 1, the liquid additive reservoir 4 comprises a rigid casing 13 made up of two injection-molded half-shells 13 a, 13 b joined together, for example by screwing or snap-fastening, and a fluid-tight flexible pouch 14 contained in the rigid casing 13. The flexible pouch 14 constitutes an additive refill. It may be positioned full in the casing 13 or alternatively positioned empty therein to make it, for example, easier to handle by reducing its mass. The flexible pouch 14 can then for example be filled using the protocols described hereinbelow once it is mounted on the vehicle.

The bottom of the flexible pouch 14 rests on the bottom of the casing 13. The bottom surface of the casing 13 is advantageously flat and smooth to avoid abrading the flexible pouch 14, particularly in the event of vibrations.

The casing 13 extends the flexible pouch 14 and is intended to be fixed to the body shell of the vehicle, preferably in a region at a moderate temperature, such as under the body shell, or in a front wing or alternatively in the luggage compartment, or even the cabin, of the vehicle.

The flexible pouch 14 is preferably made of a thermoplastic material by blow-molding.

The flexible pouch 14 has a neck fitted with means of connection to the liquid additive supply line 5.

The means of connection comprise a self-closing valve 15 with an end connector of ordinary type, here of the “Christmas tree” type, via which the valve is forcibly fitted into the neck of the pouch. The valve 15 guarantees that there will be no losses of additive before the pouch 14 is mounted on the vehicle, during phases of handling and transport, or after the pouch 14 has been removed from the vehicle for replacement, assuming that the pouch is replaced.

The self-closing valve 15 may advantageously be overmolded on the flexible pouch 14. According to an alternative form of embodiment of the invention, the pouch may be molded “onto” the self-closing valve 15, the blow-molding mold closing over the latter.

It should be noted that the electric metering pump 7 and its electronic management board may be housed in the casing 13 or offset to other locations.

According to an alternative form of embodiment of the invention, the casing 13 could be produced by blow-molding at the same time as the flexible pouch 14.

According to another alternative form of embodiment of the invention, the self-closing valve 15 could be replaced by a shut-off valve.

FIG. 2 depicts an alternative form of embodiment of the reservoir 4 which differs from that depicted in FIG. 1 essentially in that the flexible pouch 14 and the casing 13 are substantially L-shaped.

FIG. 3 depicts an alternative form of embodiment of the reservoir 4 which differs from that depicted in FIG. 1 essentially in that the side wall of the flexible pouch 14 has pleats 21 and forms a bellows giving control over how the flexible pouch deforms as it empties.

Advantageously, the side walls of the pouch 14 and of the casing 13 have a curved profile. This configuration of the side walls of the pouch 14 and of the casing 13 plays a part in holding the pouch in position during phases of stress, particularly mechanical stress (vibrations, gradients and side slopes, etc.). Note that geometries of this type and various kinds of boss, including on the main flat surfaces, are also conceivable in order further to improve the stability of the pouch.

FIG. 4 depicts an alternative form of embodiment of the reservoir 4 which differs from that depicted in FIG. 1 essentially in that the flexible pouch 14 comprises, in its upper part, a bulge 22 designed to trap any vapors of liquid additive (ammonium vapors, water vapors, etc.) so that these will not be drawn up by the pump pumping from the bottom of the pouch.

FIG. 5 depicts an alternative form of embodiment of the reservoir 4 which differs from that depicted in FIG. 3 essentially in that the liquid additive reservoir 4 comprises compression means designed to apply pressure to the flexible pouch 14.

The compression means preferably comprise a helical compression spring 23. This spring 23 applies pressure to the pouch 14, preferably via a support plate 24 situated between the spring and the pouch which will spread the thrusting force of the spring over the upper surface of the pouch 14 and thus allow uniform, controlled deformation of the pouch as it empties.

The other advantages afforded by this configuration are:

-   -   the pressure applied by the spring 23 to the pouch 14 introduces         pressure into the fluid and this greatly limits the generation         of vapors in the pouch;     -   the possibility of maintaining a pressure upstream of the pump 7         and therefore of preventing the pressure in this part of the         circuit being at atmospheric pressure or at a slight depression         as the pouch empties.

Depending on the configuration, the compression means may comprise several springs 23 designed to apply pressure to one or more plates 24. Advantageously, clips may be arranged in the upper half-shell 13 a of the casing 13 and on the support plate 24, to secure the spring 23.

FIG. 6 illustrates a method of filling the reservoir (of which only the flexible pouch 14 is depicted) according to a first implementation. Such a filling method is designed to provide perfect control over the volume of liquid introduced into the flexible pouch 14.

The filling method comprises the following steps:

-   -   equipping the supply line 5 with means of connection, such as a         three-way coupling 25 one of the paths of which is connected to         a line portion ending, for example, in a self-closing coupling         26,     -   connecting a filling assembly 27 to the supply line 5 via the         means of connection 25, 26, the filling assembly 27 comprising a         liquid additive filling line 28 advantageously provided with an         end piece designed to be inserted into the self-closing coupling         26, a pump 29 coupled to the filling line 28 and connected to a         drum 31 containing the liquid additive, and preferably a         three-way coupling 32 mounted on the filling line 28 downstream         of the pump 29 and designed to connect the filling line to a         branch line 33 connected to a purge line (not depicted in the         figures) by a relief valve 34,     -   filling the flexible pouch with liquid additive,     -   diverting all of the liquid additive supplied by the filling         line 28 to the purge line when the pressure in the line reaches         a predetermined threshold value, that is to say a pressure         designed to move the relief valve 34 into its open position.

For preference, the self-closing coupling 26 is positioned in a readily accessible location so that the filling assembly 27 can be coupled to it.

FIG. 7 depicts a selective catalytic reduction system which differs from that depicted in FIG. 1 essentially in that it is equipped with two reservoirs, namely a main reservoir 4 (of which only the flexible pouch 14 is depicted) and a secondary reservoir 41 associated with heating means (not depicted in the figure), it being possible for the secondary reservoir to be flexible or rigid. This configuration is particularly advantageous when use is being made of a reducing agent susceptible to freezing (for example AdBlue®). The secondary reservoir 41 is preferably of small volume in order to optimize its design and its cost. It is then conceivable to provide a very simple dedicated pump 42 between the two reservoirs and the task of which will simply be to transfer (for example on each restart and after each shutting down of the engine), reducing agent from the main reservoir 4 to the secondary reservoir 41. Advantageously, the secondary reservoir 41 comprises high-level detection means which are designed to stop the pump 42 and the transfer of reducing agent when the level of liquid in the secondary reservoir 41 has reached a predetermined value.

It should be noted that the method of filling the main reservoir 4 differs from that depicted in FIG. 6 in that the filling assembly 27 is coupled directly to the flexible pouch 14.

FIG. 8 depicts a selective catalytic reduction system 2 which differs from that depicted in FIG. 7 essentially in that it has no dedicated pump for transferring reducing agent from the main reservoir 4 to the secondary reservoir 41. What happens according to this embodiment is that the main feed pump 7 is used (for example by reversing the direction in which it turns) to fill the secondary reservoir 41. A nonreturn valve 43 then prevents any reflux of reducing agent from the secondary reservoir 41 to the main reservoir 4 when the feed pump 7 is operating in such a way as to supply the exhaust line with reducing agent.

As goes without saying, the invention is not restricted to the embodiments of this selective catalytic reduction system that have been described hereinabove by way of example but on the contrary encompasses all alternative forms of embodiment thereof. 

1. Liquid additive reservoir for a selective catalytic reduction system intended for treating exhaust gases of an internal combustion engine of a vehicle, comprising: a rigid casing delimiting a housing, and a fluid-tight flexible pouch mounted inside the housing of the casing and intended to contain the liquid additive, wherein the flexible pouch being designed to be connected to an exhaust line of the engine.
 2. Liquid additive reservoir according to claim 1, wherein the flexible pouch is made of thermoplastic by blow-moulding.
 3. Liquid additive reservoir according to claim 2, wherein the casing is produced by blow-moulding at the same time as the flexible pouch.
 4. Liquid additive reservoir according to claim 1, wherein the flexible pouch is made from two flexible films welded together or from two flexible half-shells welded together, the two half-shells being made by injection-moulding or thermoforming.
 5. Liquid additive reservoir according to claim 1, wherein the flexible pouch has a neck fitted with means of connection to a liquid additive supply line of the selective catalytic reduction system.
 6. Liquid additive reservoir according to claim 5, wherein the means of connection comprise a self-closing valve or a shut-off valve.
 7. Liquid additive reservoir according to claim 1, further comprising compression means designed to apply a pressure to the flexible pouch, the compression means preferably comprising a compression spring.
 8. Liquid additive reservoir according to claim 1, wherein the flexible pouch comprises, in an upper part, a bulge designed to trap liquid additive vapours.
 9. Selective catalytic reduction system intended to treat exhaust gases of an internal combustion engine of a vehicle, comprising a reservoir according to claim
 1. 10. Selective catalytic reduction system according to claim 9, further comprising: a liquid additive supply line designed to connect the flexible pouch to the engine exhaust line, an injection device coupled to the supply line and designed to inject the liquid additive into the engine exhaust line, and a selective catalytic reduction catalytic converter.
 11. Selective catalytic reduction system according to claim 10, further comprising a pump coupled to the supply line and designed to carry the liquid additive from the flexible pouch to the engine exhaust line.
 12. Method of filling a reservoir according to claim 1, the method comprising: connecting the flexible pouch to a liquid additive filling line, filling the flexible pouch with liquid additive, measuring a pressure inside the flexible pouch or in the filling line, diverting all of the liquid additive supplied by the filling line to a purge line when the measured pressure reaches a predetermined threshold value.
 13. Filling method according to claim 12, further comprising providing a relief valve between the purge line and the filling line, the relief valve being able to move between a closed position in which the purge line and the filling line are fluidically isolated to allow the flexible pouch to be filled, and an open position in which the purge line and the filling line are fluidically connected, the valve being designed to be moved automatically into the open position when the pressure inside the flexible pouch or in the filling line reaches the predetermined threshold value.
 14. Filling method according to claim 12, further comprising purging the flexible pouch before connecting the flexible pouch to the filling line, the purging step preferably being performed using a pump. 